This chronology defines the rules of the ISO calendar system.
This calendar system is based on the ISO-8601 standard, which is the
de facto world calendar.

The fields are defined as follows:

era - There are two eras, 'Current Era' (CE) and 'Before Current Era' (BCE).

year-of-era - The year-of-era is the same as the proleptic-year for the current CE era.
For the BCE era before the ISO epoch the year increases from 1 upwards as time goes backwards.

proleptic-year - The proleptic year is the same as the year-of-era for the
current era. For the previous era, years have zero, then negative values.

month-of-year - There are 12 months in an ISO year, numbered from 1 to 12.

day-of-month - There are between 28 and 31 days in each of the ISO month, numbered from 1 to 31.
Months 4, 6, 9 and 11 have 30 days, Months 1, 3, 5, 7, 8, 10 and 12 have 31 days.
Month 2 has 28 days, or 29 in a leap year.

day-of-year - There are 365 days in a standard ISO year and 366 in a leap year.
The days are numbered from 1 to 365 or 1 to 366.

leap-year - Leap years occur every 4 years, except where the year is divisble by 100 and not divisble by 400.

getCalendarType

The calendar type is an identifier defined by the
Unicode Locale Data Markup Language (LDML) specification.
It can be used to lookup the Chronology using Chronology.of(String).
It can also be used as part of a locale, accessible via
Locale.getUnicodeLocaleType(String) with the key 'ca'.

dateNow

This will query the specified clock to obtain the current date - today.
Using this method allows the use of an alternate clock for testing.
The alternate clock may be introduced using dependency injection.

isLeapYear

public boolean isLeapYear(long prolepticYear)

Checks if the year is a leap year, according to the ISO proleptic
calendar system rules.

This method applies the current rules for leap years across the whole time-line.
In general, a year is a leap year if it is divisible by four without
remainder. However, years divisible by 100, are not leap years, with
the exception of years divisible by 400 which are.

For example, 1904 is a leap year it is divisible by 4.
1900 was not a leap year as it is divisible by 100, however 2000 was a
leap year as it is divisible by 400.

The calculation is proleptic - applying the same rules into the far future and far past.
This is historically inaccurate, but is correct for the ISO-8601 standard.

eraOf

The era is, conceptually, the largest division of the time-line.
Most calendar systems have a single epoch dividing the time-line into two eras.
However, some have multiple eras, such as one for the reign of each leader.
The exact meaning is determined by the chronology according to the following constraints.

The era in use at 1970-01-01 must have the value 1.
Later eras must have sequentially higher values.
Earlier eras must have sequentially lower values.
Each chronology must refer to an enum or similar singleton to provide the era values.

This method returns the singleton era of the correct type for the specified era value.

resolveDate

Most TemporalField implementations are resolved using the
resolve method on the field. By contrast, the ChronoField class
defines fields that only have meaning relative to the chronology.
As such, ChronoField date fields are resolved here in the
context of a specific chronology.

ChronoField instances on the ISO calendar system are resolved
as follows.

EPOCH_DAY - If present, this is converted to a LocalDate
and all other date fields are then cross-checked against the date.

PROLEPTIC_MONTH - If present, then it is split into the
YEAR and MONTH_OF_YEAR. If the mode is strict or smart
then the field is validated.

YEAR_OF_ERA and ERA - If both are present, then they
are combined to form a YEAR. In lenient mode, the YEAR_OF_ERA
range is not validated, in smart and strict mode it is. The ERA is
validated for range in all three modes. If only the YEAR_OF_ERA is
present, and the mode is smart or lenient, then the current era (CE/AD)
is assumed. In strict mode, no era is assumed and the YEAR_OF_ERA is
left untouched. If only the ERA is present, then it is left untouched.

YEAR, MONTH_OF_YEAR and DAY_OF_MONTH -
If all three are present, then they are combined to form a LocalDate.
In all three modes, the YEAR is validated. If the mode is smart or strict,
then the month and day are validated, with the day validated from 1 to 31.
If the mode is lenient, then the date is combined in a manner equivalent to
creating a date on the first of January in the requested year, then adding
the difference in months, then the difference in days.
If the mode is smart, and the day-of-month is greater than the maximum for
the year-month, then the day-of-month is adjusted to the last day-of-month.
If the mode is strict, then the three fields must form a valid date.

YEAR and DAY_OF_YEAR -
If both are present, then they are combined to form a LocalDate.
In all three modes, the YEAR is validated.
If the mode is lenient, then the date is combined in a manner equivalent to
creating a date on the first of January in the requested year, then adding
the difference in days.
If the mode is smart or strict, then the two fields must form a valid date.

YEAR, MONTH_OF_YEAR, ALIGNED_WEEK_OF_MONTH and
ALIGNED_DAY_OF_WEEK_IN_MONTH -
If all four are present, then they are combined to form a LocalDate.
In all three modes, the YEAR is validated.
If the mode is lenient, then the date is combined in a manner equivalent to
creating a date on the first of January in the requested year, then adding
the difference in months, then the difference in weeks, then in days.
If the mode is smart or strict, then the all four fields are validated to
their outer ranges. The date is then combined in a manner equivalent to
creating a date on the first day of the requested year and month, then adding
the amount in weeks and days to reach their values. If the mode is strict,
the date is additionally validated to check that the day and week adjustment
did not change the month.

YEAR, MONTH_OF_YEAR, ALIGNED_WEEK_OF_MONTH and
DAY_OF_WEEK - If all four are present, then they are combined to
form a LocalDate. The approach is the same as described above for
years, months and weeks in ALIGNED_DAY_OF_WEEK_IN_MONTH.
The day-of-week is adjusted as the next or same matching day-of-week once
the years, months and weeks have been handled.

YEAR, ALIGNED_WEEK_OF_YEAR and ALIGNED_DAY_OF_WEEK_IN_YEAR -
If all three are present, then they are combined to form a LocalDate.
In all three modes, the YEAR is validated.
If the mode is lenient, then the date is combined in a manner equivalent to
creating a date on the first of January in the requested year, then adding
the difference in weeks, then in days.
If the mode is smart or strict, then the all three fields are validated to
their outer ranges. The date is then combined in a manner equivalent to
creating a date on the first day of the requested year, then adding
the amount in weeks and days to reach their values. If the mode is strict,
the date is additionally validated to check that the day and week adjustment
did not change the year.

YEAR, ALIGNED_WEEK_OF_YEAR and DAY_OF_WEEK -
If all three are present, then they are combined to form a LocalDate.
The approach is the same as described above for years and weeks in
ALIGNED_DAY_OF_WEEK_IN_YEAR. The day-of-week is adjusted as the
next or same matching day-of-week once the years and weeks have been handled.

range

All fields can be expressed as a long integer.
This method returns an object that describes the valid range for that value.

Note that the result only describes the minimum and maximum valid values
and it is important not to read too much into them. For example, there
could be values within the range that are invalid for the field.

This method will return a result whether or not the chronology supports the field.